Exercise 19.4 Assume that you are given a relation with attributes ABCD.
1. Assume that no record has NULL values. Write an SQL query that checks whether the functional dependency A → B holds.
2. Assume again that no record has NULL values. Write an SQL assertion that enforces the functional dependency A → B.
3. Let us now assume that records could have NULL values. Repeat the previous two questions under this assumption.
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